A fundamental limitation of signal processing systems intended for use in ultra-wideband applications (e.g., those used for processing signals having a frequency of several or more GHz), such as, but not limited to, passive radar direction finding receivers, is the fact that they have no a prior knowledge of the characteristics (e.g., time or direction of arrival) of a monitored signal. Because the signal of interest is typically manifested as a very narrow width time domain pulse, often barely longer than a single cycle of the transmitted carrier with little leading or trailing noise, by the time it is detected, it is gone.
One brute force proposal to solve this problem is to use an extremely fast (ultra high sampling rate) analog-to-digital converter, output data samples from which are buffered for subsequent analysis. Fundamental shortcomings of this approach include the fact that such a digitizer is extremely expensive, and processing of the digitized signal cannot be carried out in real time.